1. Field of the Invention
The present invention generally relates to a memory circuit, and particularly relates to a nonvolatile memory circuit which is capable of retaining stored data in the absence of a power supply voltage.
2. Description of the Related Art
Nonvolatile semiconductor memory devices include flash EEPROMs employing a floating gate structure, FeRAMs employing a ferroelectric film, MRAMs employing a ferromagnetic film, etc. In the case of EEPROMs, there is a need to manufacture a transistor having a special structure comprised of a floating gate. In the case of FeRAMs and MRAMs, there is a need to form a film made of a ferroelectric material and a ferromagnetic material, respectively. The need for the transistor having a special structure and the need for the film made of a special material are one of the factors that result in an increase in the manufacturing costs.
PCT/JP2003/016143, which was filed on Dec. 17, 2003, the entire contents of which are hereby incorporated by reference, discloses a nonvolatile memory cell comprised of a pair of MIS (metal-insulating film-semiconductor) transistors that have the same structure as ordinary MIS transistors used for conventional transistor functions (e.g., switching function). Namely, these memory cell transistors use neither a special structure such as a floating gate nor a special material such as a ferroelectric material or a ferromagnetic material. These MIS transistors are configured to experience an irreversible hot-carrier effect on purpose for storage of one-bit data. A difference in the transistor characteristics caused by the hot-carrier effect represents one-bit data “0” or “1”.
Japanese Patent Application Publication No. 6-76582 also discloses a nonvolatile memory cell using MIS (metal-insulating film-semiconductor) transistors. This memory cell is a latch circuit comprised of two inverters cross-coupled to each other. Two transistors having the same conduction type among the transistors constituting the latch circuit are subjected to a hot-carrier effect on purpose so as to create an imbalance in their threshold voltages. Such imbalance represents one bit data stored in the memory cell.
Generally, there is no known nonvolatile memory device that can automatically output the retained data upon the power-on. Any nonvolatile memory device known to the date that store data as a change in the electrical characteristics of a memory cell requires a read command applied thereto from an external device after the device is powered on. This is because there is a need to create a balanced state initially and to tip the balance in response to the electrical characteristics of the memory cell in order to read data from the memory cell. The above-described memory device disclosed in Japanese Patent Application Publication No. 6-76582 is no exception, and requires a read command applied thereto after the power-on.
For use in a simple system such as a small amplifier, a nonvolatile memory device is preferably provided with the function to output the retained data automatically upon the power on.